The invention concerns a two-wire cylinder dryer for drying a fiber web, notably in a paper machine.
For prior art, reference is made to the following documents:
D1: DE 36 30 570 (U.S. Pat. No. 4,694,587) PA1 D2: DE 38 18 600 PA1 D3: EP 0 472 513 PA1 2. An air carrier box is provided on the peripheral part not touched by the wire and creates in the area of the point where the web and wire leave the cylinder a vacuum zone so as to separate the web from the cylinder and subject the wire to suction. PA1 3. Provided on the opposite side of the air carrier box are blow openings for blowing preferably hot, dry air through wire into the so-called pocket.
Features which the object of the present application and the design known from D2 have in common include:
1. As it leaves each drying cylinder, the web to be dried runs at first some distance along with the respective wire, mostly up to a wire guide roll. Hence, the web is supported by the wire up to that point. Only then continues the web freely to the following cylinder of the other cylinder row. Thus, the "free web trains" (i.e., the stretches of web travel where the web is not supported), are relatively short. This precludes the risk of web flutter and web breaks.
A "pocket," as is generally known, is defined by the part of the cylinder circumference that is free of paper, by the paper travel stretches to and from this cylinder, and by the stretch of wire travel located opposite this cylinder (between two cylinders of the other cylinder row). The hot, dry air serves the swift removal of the billows emanating from the web. Across the machine width, variable air quantities can be supplied to individual zones in order to achieve a maximally uniform drying of the web across its entire width.
A problem results from the fact that the operating speed of modern paper machines is supposed to be raised to ever higher speeds (to the order of 1500 m/min and higher). This requires increasing the drying capacity per drying cylinder, and thus increasing the amount of dry air needed for billows removal. This requirement can be met only insufficiently with the prior configurations. It is either very difficult or impossible to direct the increased amounts of air to where they are needed for billows removal, namely into the pockets. With several prior designs, a major part of the supplied air fails, due to the elevated wire travel speed, to proceed through the mesh of the wire into the pockets, but is transported by the wire directly outside instead. Or, an unfavorable design (for instance according to D2) prevents a swift escape of billows-enriched air from the pockets.
The problem underlying the invention is to raise the drying capacity of the prior two-wire cylinder dryers by means of an improved air carrier box design which makes it possible to blow air quantities greater than heretofore into the pockets for billows removal and to pass them from there again outside.